Apologies - there was a subtle race condition in our code which meant that if you try to log in to the demo at the instant we're resetting our demo's data, you end up using stale demo credentials. This has now been fixed :)
Sorry about that - we have a cron job that resets the demo user data every few hours. Sometimes if you happen to catch it just at the moment we're resetting, a bug in our code means the demo credentials used by the frontend to log in to the app are stale. If you refresh, you should be okay now :)
Edit: my dev has just issued a fix for this bug, so it will not be a problem going forward!